Rendering a Water Drop in Cycles
The environment and lighting setup is very important, especially for specularly reflecting and refracting materials such as glass or water.
It looks to me like you don't have a bright enough sky. You could use a HDR sky image via the environment texture node, or the built in procedural sky texture, which can give decent results too (particularly when you don't actually see the sky directly):

For this scene I used a procedural sky texture for the background color with a strength of 4:

I also used a sun lamp to get some directional lighting (see Should I add a sun lamp when using an hdri environment?)
For the compositing setup I only used two simple glare nodes:

The de-speckle node is only to get rid of some fireflies that were appearing. Normally I'd clamp them, however in this case I wanted to leave the render un-clamped, in order to preserve any values greater than 1 for use by the glare node.

Lighting and environment aside, also ensure your glass shader is set to white. When added via the properties panel, it defaults to .8 grey, which can result in the object looking too dark.